I cannot buy that a mammalian and therapsid species could both descend that quickly from the Hebitians.

Was it ever said canonically that Cardassians were actually therapsids?

Their "scales" may be more like the ridges on a Bajoran nose. Having scale-like markings doesn't make a lizard.

A therapsid isn't actually a lizard (and neither a lizard nor a therapsid is a reptile, for that matter).

Technically, mammals and mammaliformes are the surviving descendants of the therapsids, on Earth. There were gradations of reptilianoid vs. mammaloid traits in our prehistory ranging from creatures practically indistinguishable from reptiles, and others nearly mammalian in appearance. If you look up the cynodont, for instance, it actually bears a fair resemblance to the Cardassian vole (though better-looking than the vole!): close to mammalian, but still a therapsid. Heck, if evolution had gone just slightly differently on our own planet, we ourselves could have been "Cardassians," possessed of both scales and hair.

Canon has never stated for sure what the Cardassians are, but their ability to interbreed with Bajorans makes it pretty clear that whatever they are, they are NOT reptiles. Since they don't seem quite mammalian (different metabolism, and a mix of traits that when seen on Earth, was therapsid), then therapsid seems like the best conclusion based on available evidence.